Elevated GFAP induces astrocyte dysfunction in caudal brain regions: A potential mechanism for hindbrain involved symptoms in type II Alexander disease

Rosenthal fibers (RFs) are cytoplasmic, proteinaceous aggregates. They are the pathognomonic feature of the astrocyte pathology in Alexander Disease (AxD), a neurodegenerative disorder caused by heterozygous mutations in the GFAP gene, encoding glial fibrillary acidic protein (GFAP). Although RFs have been known for many years their origin and significance remain elusive issues. We have used mouse models of AxD based on the overexpression of human GFAP (transgenic, TG) and a point mutation in mouse GFAP (knock-in, KI) to examine the formation of RFs and to find astrocyte changes that correlate with the appearance of RFs. We found RFs of various sizes and shapes. The smallest ones appear as granular depositions on intermediate filaments. These contain GFAP and the small heat shock protein, alphaB-crystallin. Their aggregation appears to give rise to large RFs. The appearance of new RFs and the growth of previously formed RFs occur over time. We determined that DAPI is a reliable marker of RFs and in parallel with Fluoro-Jade B (FJB) staining defined a high variability in the appearance of RFs, even in neighboring astrocytes. Although many astrocytes in AxD with increased levels of GFAP and with or without RFs change their phenotype, only some cells with large numbers of RFs show a profound reconstruction of cellular processes, with a loss of fine distal processes and the appearance of large, lobulated nuclei, likely due to arrested mitosis. We conclude that 1) RFs appear to originate as small, osmiophilic masses containing both GFAP and alphaB-crystallin deposited on bundles of intermediate filaments. 2) RFs continue to form within AxD astrocytes over time. 3) DAPI is a reliable marker for RFs and can be used with immunolabeling. 4) RFs appear to interfere with the successful completion of astrocyte mitosis and cell division.Electronic supplementary materialThe online version of this article (doi:10.1186/s40478-017-0425-9) contains supplementary material, which is available to authorized users.

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“…These data are in line with previous results [17] that TG mice reveal more severe pathology compared to KI mice.…”

Section: Resultssupporting

confidence: 93%

The origin of Rosenthal fibers and their contributions to astrocyte pathology in Alexander disease

Sosunov

McKhann

Goldman

2017

acta neuropathol commun

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“…In whole-cell voltage clamp recording with a K + -based solution, astrocytes exhibited a characteristic linear current-voltage (I-V) relationship membrane K + conductance or passive conductance ( Figure 2C,D). This current profile was consistent with the findings of other studies that focused on spinal grey matter [19,20]. Furthermore, these astrocytes exhibit a relatively negative resting membrane potential of −80.8 ± 3.7 mV (n = 9) and a low input membrane resistance (Rin) of 21.1 ± 7.0 MΩ (n = 5).…”

Section: Electrophysiological Properties Of Grey Matter Astrocytes Insupporting

confidence: 91%

Syncytial Isopotentiality: An Electrical Feature of Spinal Cord Astrocyte Networks

Kiyoshi

et al. 2018

Neuroglia

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Due to strong electrical coupling, syncytial isopotentiality emerges as a physiological mechanism that coordinates astrocytes into a highly efficient system in brain homeostasis. Although this electrophysiological phenomenon has now been observed in astrocyte networks established by different astrocyte subtypes, the spinal cord remains a brain region that is still unexplored. In ALDH1L1-eGFP transgenic mice, astrocytes can be visualized by confocal microscopy and the spinal cord astrocytes in grey matter are organized in a distinctive pattern. Namely, each astrocyte resides with more directly coupled neighbors at shorter interastrocytic distances compared to protoplasmic astrocytes in the hippocampal CA1 region. In whole-cell patch clamp recording, the spinal cord grey matter astrocytes exhibit passive K+ conductance and a highly hyperpolarized membrane potential of −80 mV. To answer whether syncytial isopotentiality is a shared feature of astrocyte networks in the spinal cord, the K+ content in a physiological recording solution was substituted by equimolar Na+ for whole-cell recording in spinal cord slices. In uncoupled single astrocytes, this substitution of endogenous K+ with Na+ is known to depolarize astrocytes to around 0 mV as predicted by Goldman–Hodgkin–Katz (GHK) equation. In contrast, the existence of syncytial isopotentiality is indicated by a disobedience of the GHK predication as the recorded astrocyte’s membrane potential remains at a quasi-physiological level that is comparable to its neighbors due to strong electrical coupling. We showed that the strength of syncytial isopotentiality in spinal cord grey matter is significantly stronger than that of astrocyte network in the hippocampal CA1 region. Thus, this study corroborates the notion that syncytial isopotentiality most likely represents a system-wide electrical feature of astrocytic networks throughout the brain.

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“…The neurological abnormalities and early death of AxD can be recapitulated in mice and flies by overexpression not only of mutant forms of GFAP but also of normal GFAP [18, 45–47]. In AxD, a reduction in the level of the GLT1 glutamate transporter in vulnerable brain regions has been suggested to affect glutamate clearance leading to excitotoxic death of neurons [48–50]. Similarly, in MeCP2-Tg mice, the increased expression of GFAP and reduction in GLAST is most pronounced in the MeCP2-Tg cortex and hippocampus, where neuronal loss occurs subsequently.…”

Section: Discussionmentioning

confidence: 99%

Elevated MeCP2 in Mice Causes Neurodegeneration Involving Tau Dysregulation and Excitotoxicity: Implications for the Understanding and Treatment of MeCP2 Triplication Syndrome

et al. 2018

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Expression of MeCP2 must be carefully regulated as a reduction or increase results in serious neurological disorders. We are studying transgenic mice in which the MeCP2 gene is expressed at about three times higher than the normal level. Male MeCP2-Tg mice, but not female mice, suffer motor and cognitive deficits and die at 18-20 weeks of age. MeCP2-Tg mice display elevated GFAP and Tau expression within the hippocampus and cortex followed by neuronal loss in these brain regions. Loss of Purkinje neurons, but not of granule neurons in the cerebellar cortex is also seen. Exposure of cultured cortical neurons to either conditioned medium from astrocytes (ACM) derived from male MeCP2-Tg mice or normal astrocytes in which MeCP2 is expressed at elevated levels promotes their death. Interestingly, ACM from male, but not female MeCP2-Tg mice, displays this neurotoxicity reflecting the gender selectivity of neurological symptoms in mice. Male ACM, but not female ACM, contains highly elevated levels of glutamate, and its neurotoxicity can be prevented by MK-801, indicating that it is caused by excitotoxicity. Based on the close phenotypic resemblance of MeCP2-Tg mice to patients with MECP2 triplication syndrome, we suggest for the first time that the human syndrome is a neurodegenerative disorder resulting from astrocyte dysfunction that leads to Tau-mediated excitotoxic neurodegeneration. Loss of cortical and hippocampal neurons may explain the mental retardation and epilepsy in patients, whereas ataxia likely results from the loss of Purkinje neurons.

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Elevated GFAP induces astrocyte dysfunction in caudal brain regions: A potential mechanism for hindbrain involved symptoms in type II Alexander disease

Cited by 27 publications

References 65 publications

The origin of Rosenthal fibers and their contributions to astrocyte pathology in Alexander disease

The origin of Rosenthal fibers and their contributions to astrocyte pathology in Alexander disease

Syncytial Isopotentiality: An Electrical Feature of Spinal Cord Astrocyte Networks

Elevated MeCP2 in Mice Causes Neurodegeneration Involving Tau Dysregulation and Excitotoxicity: Implications for the Understanding and Treatment of MeCP2 Triplication Syndrome

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